How to count element frequencies in an array

Swift version: 5.2

Paul Hudson @twostraws May 28th 2019

If you have an array containing various elements and you want to count how often each item appears, you can do so by combining the map() method with a Dictionary initializer.

First, create an array of items:

let items = ["a", "b", "a", "c"]

Second, convert that to an array of key-value pairs using tuples, where each value is the number 1:

let mappedItems = items.map { ($0, 1) }

Finally, create a Dictionary from that tuple array, asking it to add the 1s together every time it finds a duplicate key:

let counts = Dictionary(mappedItems, uniquingKeysWith: +)

That will create the dictionary ["b": 1, "a": 2, "c": 1] because dictionaries are not stored in order – as you can see, it tells us that “a” appeared twice, while the other two appeared once.